Patent-related reference 1: JP-A-5-87934 (87934/1993)
Patent-related reference 2: JP-A-2004-137489
Because of excellent transmissivities over wide wavelength ranges, specific natures, and the like, fluoride crystals have been used as optical materials for optical lithography, crystals for all-solid-state ultraviolet/infrared laser, ultraviolet region window materials, scintillators, and the like. Among them, rare earth fluorides have been widely used as a scintillator, a dopant of an optical fiber amplifier, host and dopant of a solid-state laser material, and the like, by utilizing natures of rare earth elements, and by utilizing Ce3+ in case of CeF3. Further, since rare earth fluoride crystals each have a higher density in itself, these crystals can be expected to be utilized as scintillators, host materials of solid-state laser, and the like.
Rare earth fluorides are classified into those which exhibit primary phase transition at or below melting points thereof, respectively, and those which do not. LaF3, CeF3, PrF3, and NdF3 each have a Tysonite structure, and TbF3, DyF3, and HoF3 each also have a β-YF3 structure, and they do not exhibit primary phase transition in a range from a room temperature to each melting point, and are thus possible in single crystal growth from a melt. Meanwhile, the other rare earth fluorides (SmF3, EuF3, GdF3, ErF3, TmF3, YbF3, LuF3, YF3, and ScF3) each exhibit primary phase transition (orthohombic<->hexagonal or orthohombic<->trigonal) at a temperature between a room temperature and each melting point, so that conduction of crystal growth from a melt leads to occurrence of cracks in the course of cooling. Otherwise, only polycrystals are obtained, thereby considerably obstructing development of a novel single crystal material. It is particularly regretful for a LuF3 crystal having the highest density, that LuF3 in itself is impossible in single crystal growth from a melt such as by a micro-pulling-down method, a Czochralski method, a Bridgman method, a zone melt method, or an Edge-defined Film-fed Growth (EFG) method, due to the problem of primary phase transition.
In a positron emission tomography (PET) apparatus, gamma rays (annihilation gamma rays: 511 eV) are to be detected by coincidence, so that there have been adopted scintillation detectors each having a higher sensitivity and being capable of obtaining a high speed response. As detector characteristics, there are required a higher time resolution for a higher counting rate property, removal of accidental coincidence noises, and the like, and there is desired an excellent energy resolution for eliminating scattered radiation from an interior of a body.
Thus, scintillators satisfying such requirements and suitable for detectors, are each required to have a higher density and a larger atomic number (a higher photoelectric absorption ratio) from a standpoint of detection efficiency, and a higher emission amount and a shorter fluorescence lifetime (fluorescence decay time) from standpoints of a required high speed response, a higher energy resolution, and the like. Further, since many scintillators are required to be densely arranged in fine and elongated configurations in recent multi-layered and high resolutive systems, important selection factors of scintillators include operability, processability, and cost.
Although Tl:NaI has been most typically used for a scintillation detector by virtue of a higher emission amount at a relatively low cost thereof, improved sensitivities of detectors can not be expected due to the lower density of Tl:NaI and the same is difficult in handling due to deliquescency thereof, so that Tl:NaI has been replaced by Bi4Ge3O12 (BGO).
BGO has a wavelength of 490 nm, a refractive index of 2.15, and a density of 7.13 g/cm3 which is two times as dense as that of Tl:NaI, so that BGO has a higher linear energy absorption coefficient for gamma rays. Further, although Tl:NaI has hygroscopicity, BGO does not have hygroscopicity thereby advantageously exhibiting a higher processability. It is noted that BGO has such defects that: the same has a fluorescence conversion efficiency which is 8% of that of Tl:NaI, so that an optical output of BGO for gamma rays is smaller than that of Tl:NaI; and BGO exhibits an energy resolution of 15% for gamma rays of 1 MeV, whereas Tl:NaI exhibits 7%. Moreover, its fluorescence decay time is relatively long on the order of 300 nsec, so that there is desired development of a scintillator having a shorter fluorescence lifetime so as to be utilized in devices such as PET, SPECT, and the like for photon counting.
Since BaF2 has an extremely fast component (˜0.8 nsec) in an ultraviolet region (near ˜220 nm), there can be expected a higher time resolution. Thus, the same has been regarded as a leading candidate material for a scintillator of a PET of a time of flight (TOF) type utilizing a flight time difference. However, the obtained time resolution was on the order of 400 psec (calculated as about 6 cm of positional resolution), thereby failing to reach usage of a time information for direct imaging, while merely bringing about an improved signal-to-noise ratio and counting rate characteristics. Also concerning a photomultiplier tube (PMT), there is required an expensive window material in case of transmission of wavelengths near 220 nm, thereby leading to an increased cost of device. Moreover, BaF2 exhibits a detection efficiency which is significantly inferior to that of BGO such that BaF2 is low in resolving power and sensitivity characteristics, resulting in that development of PET's specialized in TOF types are rarely conducted now.
Ce:Gd2SiO5 (Ce:GSO) has been developed in Japan, which is a high performance scintillator well balanced in a density (6.71 g/cm3), a light amount (two times that of BGO), a response speed (30 to 60 nsec), and a radiation resistance (>105 gray), though the same is slightly inferior to BGO in detection sensitivity. However, Ce:GSO is problematic, since the same also has a slightly delayed rising-up, positive-hysteresis (property by which a light amount is increased depending on irradiation) relative to radiation, and a strong cleavage property.
Currently, the most-advanced scintillator crystal is Ce-added Lu2SiO5 (Ce:LSO), which exhibits excellent scintillator characteristics such as a higher density (˜7.39 g/cm3), a shorter fluorescence lifetime (about 50 nsec), and a higher emission amount (three or more times that of BGO). The LSO crystal can be fabricated by the Czochralski method, so that the same provides a market of several hundreds of millions of dollars, mainly including companies in the United States such as CTI Molecular Imaging Inc. (CTI), Crystal Photonics Inc. (CPI). In turn, the LSO crystal has problems of increased production and working costs and a deteriorated yield, due to its characteristics such as an excessively higher melting point of 2,150° C., a higher anisotropy in linear expansion coefficients, and the like. Although there is typically used a metal called iridium (Ir) as a crucible material for melt growth of an oxide single crystal having a higher melting point, temperatures exceeding 2,000° C. are closer to the softening temperature of Ir, so that extremely severe temperature control is required for production of LSO crystal. In addition, since the usable lifetime of an Ir crucible is also short, extensive costs for recasting crucibles also become a severe burden to manufacturers. Further, there are required a high-frequency oscillator and a higher output for realizing such ultra-high temperatures, thereby generally increasing a running cost.
Meanwhile, concerning the Ce:GSO and Ce:LSO having been used as scintillator-oriented emission materials, although larger inclusion amounts of Ce acting as an emission element lead to higher emission amounts, inclusion amounts exceeding several % lead to significant concentration quenching, thereby failing to exhibit a scintillator effect.
Further, Ce is in a large size next to La among rare earth ions, and is significantly large as compared to representative rare earth ions (Y, Gd, Lu), resulting in that Ce exhibits an effective segregation coefficient largely deviated from “1”. Namely, there is inevitably caused a compositional fluctuation of Ce along a growth direction thereof. This phenomenon becomes a cause of changing physical properties such as fluorescence decay time, emission amount, and the like, thereby causing a significant problem upon usage in a PET and the like of a high accuracy specification.
There have never been attained any materials based on fluorides, which simultaneously meet excellent scintillator characteristics, i.e., a higher density, a shorter fluorescence lifetime, and a higher emission amount.
Even under such circumstances, there is currently desired development of a next generation scintillator which has a higher energy absorption coefficient, is non-hygroscopic, and is high in energy resolution, time resolution, and the like, at a due cost.
Further, although Pr, Ce, F:Gd2O2S ceramic scintillators described in the patent-related reference 1 are high in emission efficiency, single crystals in practical sizes can not be fabricated while light-transmitting ceramics can be narrowly fabricated. The light transmittance is about 60%/mm, so that fluorescence emitted in the scintillator does not reach a photodiode in a full amount, thereby deteriorating a sensitivity. Further, it has a problem of a complicated production process and a higher cost.
Meanwhile, described in the patent-related reference 2 is a technique aiming at providing a radiation detection material having a higher resolution than conventional materials and having an ultra high-speed response.
Its configuration resides in a fluoride single crystal material for detecting radiation characterized in that the material is a rare earth fluoride single crystal represented by REF3 (RE is at least one selected from Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).
There is also described a fluoride single crystal material for detecting radiation characterized in that the material is represented by RE1−xRxF3 (x<0.5) such that the material includes, together with the above-mentioned RE, at least one element RE selected from rare earth elements other than the RE and La.
However, the patent-related reference 2 fails to describe any concrete combinations of RE and R. Further, only Lu is referred to as RE in its embodiments.
Additionally, since it is difficult to grow a single crystal from a melt due to the problem of primary phase transition as clearly described in item [0004] of the present specification, this patent-related reference has attempted a single crystal growth from a solution including potassium fluoride or the like added thereto as a flux. However, the crystal growth from a solution has undesirable features from a standpoint of optical crystal, such as a slower crystal growth rate, inclusion of the flux into the crystal as impurities, and the like.
Further, the emission wavelength of Ce3+ in a cerium-substituted lutetium fluoride crystal of the patent-related reference 2 is 310 nm, which leads to a deteriorated sensitivity in case of adoption of a typical PMT made of quartz. When it is desired to give importance to sensitivity, there is thus required a specific PMT, thereby also causing a problem of cost. Moreover, although the cerium-substituted lutetium fluoride crystal has a higher emission wavelength as compared with BaF2, the fluorescence lifetime of the former is 23 nsec, so that usage thereof for a TOF type is difficult. Additionally, although the fluorescence lifetime of a crystal of lutetium fluoride itself is described to be 0.8 nsec, the emission amount thereof is not described at all.